Thermoplastic elastomers (TPEs) are phase-separated systems in which at room temperature, the dispersed phase is rigid (i.e. glassy and/or crystallized) and the matrix phase is soft and elastomeric. The rigid domains act as crosslinks for the elastomeric phase, which is analogous to chemical crosslinks in conventional vulcanized rubbers. However, the crosslinks of the rigid domain is often referred to as physical crosslinking because flow can occur when the TPE is processed at high temperatures. Usually, TPEs have two service temperatures due to this phase-separated structure. The lower service temperature depends on the glass transition temperature of the elastomeric phase, Tg1, whereas the upper service temperature depends on the glass transition temperature of the glassy phase, Tg2, and/or its melting point, Tm, if crystallizable. Thus, the application temperature window of the TPE is represented by ΔT=Tg2−Tg1 or Tm−Tg1.
A plasticizer can be used to reduce the stiffness and/or Tg of a polymer. As an added benefit, a plasticizer can also improve the processability of the polymer. Plasticizers are liquids of low volatility and good thermal stability. Plasticizers are typically organic esters, e.g., phthalates, trimellitates, and adipates for plasticizing poly(vinyl chloride) (PVC). Typically, plasticizer efficiency is proportional to the Tg of the polymer to be plasticized. In other words, the efficiency is greater when the polymer has a higher Tg.
Hydrogenated styrenic block copolymers, such as those having a saturated ethylene-butene-1 mid-block (i.e., SEBS), possess good thermal stability. However, their use in certain processes, such as thermoforming operations, is limited because of poor processability. This is thought to be a result of the high incompatibility of the styrene end-block and the EB mid-block even in the melt state. Therefore, despite its stability to high temperatures, ultraviolet light, or ozone, SEBS is seldom used alone. Some common compounding ingredients for SEBS are oils, polyethylene, polypropylene, and fillers. Oils make the products softer and also act as processing aids. Paraffinic oils are preferred because they are more compatible with the EB center block. Aromatic oils are generally avoided because they intrude into and plasticize the polystyrene domains. Blends with as high as 90% oil (5% SEBS and 5% wax) have been used as cable filling compounds, which occupy the voids in “bundled” telephone cables and prevent water seepage. Other applications include toys, hand exercising grips, etc.
Polypropylene (PP) is the preferred polymeric additive to SEBS because it ameliorates the aforementioned processability problem when used in conjunction with process oils. In injection molding or extrusion molding of useful parts, the PP/SEBS/oil mixture forms two co-continuous phases. The high melting-point PP phase enhances both the solvent-resistance and heat-resistance of the compounds. Additionally, the presence of SEBS improves the impact resistance of PP, especially at low temperatures. Normally this would be accompanied by a loss in clarity because the added polymer forms a separate phase with a different refractive index. However, PP/SEBS blends are nearly as transparent as neat PP, probably because of matched refractive indices. In some applications, LLDPE is added to SEBS. Blends of LLDPE/SEBS with PP also retain the clarity of the neat PP and show improved impact resistance. These blends are mostly used to make blown film, where they have improved impact resistance and tear strength, especially in the seal area.
As a generalization, conditions for processing compounds based on SEBS by injection molding, blow molding, extrusion, etc. are processed under conditions suitable for PP. However, difficulties exist in using high levels of process oil in the blends to achieve lower values of hardness because the oil tends to bloom to the surface (“surface blooming”) and/or emit from the polymers during service. Oils also have a yellowish appearance which detracts from the optical properties in the final product. Further, oils tend to emit a distinct odor which detracts from its use in closed or contained environments, such as automotive interiors.
Typically, mineral oils or synthetic oils are added as the processing oil or plasticizer to block copolymers including SEBS polymers to improve processability. Mineral oils are any petroleum-based oil that is derived from petroleum crude oil and subjected to refining steps, such as distillation, solvent processing, hydroprocessing, and/or dewaxing to achieve the final oil. This also includes petroleum-based oils that are extensively purified and/or modified through severe processing treatments. Examples of commercially available mineral oils include but are not limited to Drakeol from Penreco (USA), Paralux from Chevron (USA), Sunpar from Sunoco (USA), Plastol and Flexon from ExxonMobil (USA), Shellflex from Royal Dutch Shell (UK/Netherlands), and Diana from Idemitsu (Japan).
Other block copolymer blends have been proposed to improve processability and maintain thermal stability. For example, U.S. Pat. No. 4,904,731 teaches polymeric compositions of a C2-C10 olefin polymer, a hydrogenated block copolymer, and an LLDPE, useful for shaped structures having good clarity and good impact strength. U.S. Pat. No. 5,925,707 discloses oil gel compositions of styrene block copolymers, oil, and optionally a polyolefin wax and/or liquid extender. WO 02/31044 discloses a composition of SEBS, polypropylene, and a polydecene having a molecular weight of from about 400 and 1000 g/mol to make flexible products. Other references include U.S. Pat. No. 4,132,698; U.S. Pat. No. 4,960,820; WO 01/18109; WO 2004/014998; and EP 0300689.
There is a need, therefore, for an elastomeric block copolymer having improved flexibility after prolonged heat-aging and/or cooling with less surface blooming.